Half-bridge inverters are widely known for the operation of light sources. The half-bridge inverter is fed with a supply voltage, which is a DC voltage. For light sources whose light flux responds only slowly to the electrical energy provided, the DC voltage may be pulsed without impairing the quality of the light. Halogen incandescent lamps represent such a light source. Half-bridge inverters for halogen discharge lamps are therefore generally fed with a rectified mains voltage as the supply voltage, without any smoothing being provided.
It is also widely known that half-bridge inverters for halogen incandescent lamps are embodied as self-commutated inverters for cost reasons. In this context, self-commutated means that a drive signal for electronic switches of the half-bridge is taken from an output circuit. In what follows, the term half-bridge inverter is always intended to mean a self-commutated half-bridge inverter. It consists essentially of the series circuit of an upper electronic switch and a lower electronic switch, which are joined at a half-bridge midpoint and are connected between a supply voltage and a ground potential.
The commutation of the half-bridge inverter has to be started by a start circuit. This is necessary after each mains half-wave since the commutation is broken off when there is a low instantaneous mains voltage. The start circuit consists essentially of a start capacitor and a trigger element. As soon as the voltage at the start capacitor exceeds a trigger threshold, a start pulse is initiated; this means that the trigger element connects the start capacitor to the control electrode of the lower electronic switch. The lower electronic switch is therefore turned on and the commutation of the half-bridge inverter commences. The start capacitor must deliver enough energy for the lower electronic switch to remain reliably turned on for long enough.
So that the circuit is not destroyed in the event of malfunction, and no damage is incurred due to an incorrect load, the circuits in question contain a switch-off device. The switch-off device has an input and output. It is configured and connected so that it discharges the start capacitor if a switch-off signal is applied to the input. The commutation of the half-bridge inverter breaks off at the next mains voltage zero crossing. The switch-off device prevents a restart.
The prior art concerning a circuit with a switch-off device for the operation of light sources will be explained below with reference to FIG. 1. The series circuit of an upper electronic switch T1 and a lower electronic switch T2 is connected between a terminal for a supply voltage, which is denoted by a + sign, and a ground potential. The interconnection node M forms a half-bridge midpoint. The electronic switches are presented as NPN bipolar transistors. It is nevertheless possible to use other electronic switches, for example MOSFETs or IGBTs. An optional freewheel diode is respectively depicted in parallel with each electronic circuit.
The series circuit of two coupling capacitors C7 and C8 is connected in parallel with the series circuit of T1 and T2. There is a second node N2 at the interconnection point between C7 and C8. The half-bridge inverter, which consists essentially of T1, T2, C7 and C8, delivers an AC voltage of high frequency compared with a mains voltage between the second node N2 and the half-bridge midpoint M. The series circuit of the primary winding of a feedback transformer Tr1 and a load Lp is connected to this AC voltage. One coupling capacitor C7 or C8 may be omitted.
The load is represented by a resistor with the reference Lp. In the simplest case, the load may consist merely of an incandescent lamp. Alternatively, low-voltage halogen incandescent lamps may be connected up via a transformer.
By the primary winding, the feedback transformer Tr1 picks up a load current and couples it back via two secondary windings respectively to control electrodes of T1 and T2. A feedback circuit is thereby closed, so that a self-commutating half-bridge inverter is obtained.
A start circuit is formed by a start capacitor C3, a resistor R1 and a trigger element DIAC. C3 and R1 are connected in series between the supply voltage and the ground potential. The DIAC joins the interconnection point of C3 and R1 to the control electrode of the lower electronic switch T2. Via R1, C3 is charged with the supply voltage. If the voltage reaches a value which is more than the threshold voltage of the DIAC, i.e. typically 33 V, then C3 sends a start pulse to the control electrode of T2.
Once the half-bridge inverter is commutating, it is necessary to ensure that no further start pulses arrive from the start circuit, since these would perturb the commutation in progress. In fact, a start pulse while the upper electronic switch is turned on would actually destroy the half-bridge since a so-called cross current is set up. In the prior art according to FIG. 1, this problem is resolved by a diode D11 which joins the start capacitor C3 to the half-bridge midpoint M. As soon as the lower electronic switch is turned on, the start capacitor C3 discharges through the diode D11.
The series circuit of a first limiter diode D7 and a second limiter diode D8 is connected between the supply voltage (+) and the ground potential, the limiter diodes being joined to a third node (N3). The third node N3 is joined to the second node N2. The limiter diodes are intended to prevent the voltage at the coupling capacitors C7 and C8 from changing polarity, so that the potential at the second node N2 does not exceed the supply voltage or fall below the ground potential, in the event of an elevated load current.
In the prior art according to FIG. 1, a switch-off device consists of resistors R3-R6, a capacitor C4, a diode D3 and a transistor T3. The voltage at the second node N2 is used as a switch-off signal. The amplitude of the AC voltage component of the voltage at the second node N2 is a measure of the load current. The circuit is intended to be disabled if the load current exceeds a predetermined limit value.
The voltage at the second node N2 is joined via a fifth capacitor C5 to an input E of the switch-off the device. The AC voltage component of the voltage at N2 is filtered out by C5 and made available at a grounded third resistor R3. The voltage at R3 is rectified by a third diode D3 and charges a fourth capacitor C4 via a fourth resistor R4.
R4 and C4 form a lowpass filter. It is used so that an elevated load current does not lead to a switch-off process until after a predetermined time. This is necessary since, by their very nature, cold filaments of an incandescent lamp lead to an elevated load current. Typically, halogen incandescent lamps do not reach their rated value until about 0.1 sec after switching on.
Connected in parallel with C4, there is a fifth resistor R5 which discharges C4 again after an elevated load current. The voltage at C4 is fed to the base of the transistor T3 via a voltage divider, formed by the resistors R6 and R7. For cost reasons, T3 is generally embodied as a bipolar transistor. If T3 is driven in the event of malfunction, then it short circuits the start capacitor C3 via its collector-emitter path. The voltage divider formed by R6 and R7 adapts the voltage level at C4 to the switch-on threshold of T3. R7 may optionally be omitted.
FIG. 1 represents the way in which a switch-off signal is taken from the node N2 between the coupling capacitors C7 and C8. As an alternative, the current in the lower electronic switch T2 may also be used as a switch-off signal. To this end, a measuring resistor R2 is connected in series with the lower electronic switch T2. The voltage at R2 is fed to the input E of the switch-off device. C5 may be omitted.
One problem with the circuit according to the prior art is the dimensioning of the lowpass filter consisting of R4 and C4. In order to achieve the necessary delay and make sufficient energy available for driving the transistor T3, the capacitor C4 in the prior art typically has values of from 10 to 47 microfarads. These values mean that C4 is very large and expensive compared to other components of the circuit. For these capacitances, furthermore, it is customary to use electrolytic capacitors which age, work only in a restricted temperature range and are difficult to mount in mass production.